The disclosures herein relate generally to seismic ocean bottom cables and more particularly to locating the position of each hydrophone on the ocean bottom.
One of the problems in seismic under-sea exploration is to locate the receivers (hydrophones and or geophones) on the ocean bottom with respect to a source of sound. The source of sound is located with respect to an electronic navigation system. The most commonly used navigation system is the differential GPS (Global Positioning System) using satellites.
Existing systems now use an additional acoustic positioning system to locate a towed cable and receivers (hydrophones). These systems usually do not have an acoustic sensor at each hydrophone seismic sensor. The frequency used is in the range of 30 KHz to above 100 KHz. A related problem in bottom cable exploration is that if the sensor sinks into the mud at the ocean bottom, the high frequency acoustic source may not penetrate the mud and may not work.
One such system is described in U.S. Pat. No. 5,668,775 which includes seismic equipment arranged to transmit and receive signals. The equipment is included in streamers as part of a marine seismic tow. Furthermore, the equipment uses several acoustic transmitters and operates with acoustic receivers separated by a known distance. Also, the location of at least one point in a spatial network is defined by relating the network to at least one conventionally determined geographical position.
Therefore, what is needed is a method and apparatus for determining the position of seismic equipment placed on the ocean bottom rather than being towed behind a boat such that the cables are in a fixed position on the ocean bottom and a recording boat is maintained in position by anchoring or by directional thrusters.